In recent years, a technique by which transistors are manufactured using an oxide semiconductor as a semiconductor material and the transistors are applied to semiconductor circuits, ICs, electro-optical devices, electronic appliances and the like has attracted attention.
For example, Patent Document 1 and Patent Document 2 disclose a technique with which a thin film transistor (also referred to as a TFT) is manufactured using a semiconductor thin film (a thickness of about several nanometers to several hundreds nanometers) including zinc oxide, an In—Ga—Zn—O-based oxide semiconductor, or the like over a substrate having an insulating surface, and such a TFT is used for a switching element of an image display device.
The conventional transistor is manufactured mainly using a semiconductor material such as amorphous silicon or polycrystalline silicon. The TFT using amorphous silicon has a low electric field-effect mobility but can relatively easily respond to an increase in size of a manufacturing substrate such as a glass substrate. On the other hand, the TFT using polycrystalline silicon has a high electric field-effect mobility, but needs a crystallization step such as laser annealing and is not always adaptable to an increase in size of a manufacturing substrate such as a glass substrate.
In contrast, a TFT in which a channel formation region (also referred to as a channel region) is provided in an oxide semiconductor can have higher field-effect mobility than a TFT using amorphous silicon. Further, an oxide semiconductor film can be formed by a sputtering method or the like. A manufacturing process of the TFT using an oxide semiconductor is simpler than that of a TFT using polycrystalline silicon and easily responds to an increase in size of a manufacturing substrate.
An oxide semiconductor which can be used for a high-performance transistor over a glass substrate, a plastic substrate, or the like is expected to be applied to display devices such as a liquid crystal display, an electroluminescent display (also referred to as an EL display), and electronic paper.
In particular, there is a trend in an active matrix semiconductor device typified by a liquid crystal display device towards a larger screen, e.g., a 60-inch diagonal screen, and further, the development of an active matrix semiconductor device is aimed even at a screen size of a diagonal of 120 inches or more. In addition, a trend in resolution of a screen is toward higher definition, e.g., high-definition (HD) image quality (1366×768) or full high-definition (FHD) image quality (1920×1080), and a so-called 4K Digital Cinema display device, which has a resolution of 3840×2048 or 4096×2180, is also urgently developed.
As a display device has a larger size and a higher definition, the number of pixels needed for the display device is significantly increased. As a result, writing time for one pixel is required to be shortened, and thus a transistor arranged in a pixel is required to have high speed operation characteristics, large on current, and the like. In the meantime, a problem of energy depletion in recent years has caused demand for a display device whose power consumption is suppressed. Therefore, a transistor is also required to have low off current and suppressed unnecessary leakage current.
As described above, transistors having high ratio of on current to off current are desired. A technique of a transistor using an oxide semiconductor, in which the ratio of on current to off current is increased to about 103, is disclosed in Patent Document 3.
Increase in screen size or definition tends to increase wiring resistance in a display portion. Increase in wiring resistance causes delay of signal transmission to an end portion of a signal line, drop in voltage of a power supply line, or the like. As a result, deterioration of display quality, such as display unevenness or a defect in grayscale, or increase in power consumption is caused.
In order to suppress increase in wiring resistance, the technique by which a low-resistance wiring layer is formed using copper (Cu) has been considered (for example, see Patent Document 4 or 5).